In each country there is a grid code which sets out the various standards that a power generator must meet to connect to the grid. The same applies to wind power. There are various requirements, one of which is to support the grid voltage during a grid fault. For example when the voltage at the point where the wind farm connects to the grid (point of common connection PCC) drops below 0.9 p.u. (90% of the nominal voltage) the wind turbines might be expected to inject a certain amount of reactive current.
For wind farms, this requirement is often met using capacitor banks and static synchronous compensators (STATCOMs), especially if the turbine is not a variable speed wind turbine. It has been seen that even for variable speed wind turbines certain amount of compensation equipment (STATCOMs and capacitor banks) are required. However, this compensation equipment is expensive and adds to the cost of Balance of Plant (BoP) equipment and which is required over and above the wind turbine generators (WTGs). Further, complex control strategy is required to meet the reactive power requirement at the PCC using the additional compensation.
In some cases the wind turbine controller may enter a fault mode when a low grid voltage condition occurs at the turbine terminals or in the grid. The fault mode may include injecting reactive current into the grid to support the grid voltage. However due to the significant impedances in the system external to the WTG, when the WTG starts injecting a reactive current the terminal voltage changes relative to voltages further along into the grid i.e. the actual voltage, and thus such a compensation from the turbine may not be accurate or reliable. Also there may be a problem where the WTG gets stuck in the fault mode leading to tripping of the WTG eventually. The same can be explained when the turbine is absorbing reactive power due to a high voltage event in the grid.